A charge coupled device (CCD) imager contains an array of light detecting sites, referred to herein as "pixels", which accumulate charge depending on the light energy projected onto them. After some charge accumulation time, the charges in the light detecting pixels are transferred to a charge shifting structure so that the charges may be shifted out of the CCD and measured by signal processing circuits to derive a signal representative of the image projected onto the CCD. Due to manufacturing variability in the CCD structure, light source non-uniformities, dust or other contaminants in the optical path which projects the image onto the CCD, or for other reasons, the response of the individual light detecting pixels can be non-uniform from pixel-to-pixel. It is well known to compensate for these pixel-to-pixel variations in the charge measuring process, for example by multiplying the output value of the signal for each pixel by a gain value and adding an offset value. These compensating values are usually determined on a pixel-to-pixel basis during a calibration process and stored for use during image sensing. The result is to make the responsiveness of all the pixels substantially equal.
In analog CCD signal processing circuits, V.sub.dark represents the signal from a CCD pixel which is not exposed to light. Hence, V.sub.dark is the lowest value limit of the signal from the CCD since the signal cannot go beyond V.sub.dark. If V.sub.dark is nominally at zero volts, then the signal in the CCD signal processing circuits will always be either positive or negative. This is a so-called unipolar circuit because all signal processing takes place in a single quadrant. In this case, the application of a compensating gain value will not affect the nominal V.sub.dark level, because zero volts multiplied by any value remains zero volts. On the other hand, it may be desirable to make V.sub.dark and the maximum signal level, V.sub.sat, equal and opposite in polarity in order to make optimum use of the operating ranges of the circuit devices in the signal processing path. This is a so-called bipolar circuit because all signal processing takes place in two quadrants. In this case, however, a problem arises in that a change in the compensating gain value will cause an unwanted change in V.sub.dark from its nominal value. There is therefore a need to provide a CCD bipolar signal processing circuit that compensates for this adverse effect caused by gain changes introduced for signal compensation purposes. The present invention satisfies this need.